[One of the great advantages of going systematically through the Ten Commandments as a whole, rather than dipping in and out of them, is that by the time you reach the seventh one, everybody knows what to expect; nobody can accuse you of being obsessed with the particular sins it is denouncing, since you are following God in denouncing all sin. There are civilisations that have struggled with the command not to kill, but welcomed the commands against sexual immorality; ours, speaking for the UK at least, rejoices in the command not to kill, but finds those against sexual immorality constraining, outdated or worse. But Heidelberg just chunters on. God says you shouldn't do it, so don't.

And, as ever, the Catechism gives reasons, not just imperatives. God hates unchastity (which, we should note, does not mean "sexual activity" but "sexual activity outside of marriage"), so we should "thoroughly detest it", whether we are married or not. "We are temples of the Holy Spirit, body and soul, and God wants both to be kept clean and holy." Consequently, we should not only abstain from sexual immorality; we should abstain from anything that might cause it (actions, looks, talk, thoughts, desires), and from anything that might cause something that might cause it (pornography is an obvious example, but to be honest, so are an awful lot of adverts, TV programmes, conversations and jokes). Yes, that's a big ask. No, it isn't easy. But that's why the gospel, so beautifully expounded in the first half of the Catechism, is so vital.]